The present invention provides novel compounds which are pharmacologically useful as hypolipidemic drugs (e.g., those drugs which are helpful in reducing serum levels of cholesterol). More specifically, the compounds of the present invention are orally active hypolipidemic agents which promote their cholesterol lowering effects through their ability to inhibit the activity of the enzyme .delta.-hydroxy-.delta.-methyl-glutaryl Co-enzyme A (HMG CoA) and thus inhibit the formation of serum cholesterol. HMG CoA is a substance which controls the rate at which cholesterol is synthesized in hepatocytes (e.g., cells of mammalian liver, which are thought to be one of the two principle in vivo sources of serum cholesterol). The present invention also relates to novel pharmaceutical compositions comprising one or more of the active compounds of the invention in combination with suitable pharmaceutical carriers as well as methods of using such compounds and pharmaceutical compositions thereof in the treatment, prevention, or mitigation of hyperlipoproteinemia, including specifically type II hyperlipoproteinemia, which is characterized by an excess of serum low density lipoprotein (LDL). Thus, the compounds of the instant invention are useful to inhibit sterol biosynthesis in individuals predisposed to familial type hypercholesterolemia. The significance of such compounds is widely recognized, e.g. Breslow et al., Biochim. Biophys. Acta, 398, 10 (1975); Betheridge et al., rit. Med. J., 4,500 (1975); and Brown et al., J. Biol. Chem. 249, 7306 (1974). In addition, the compounds can be used in in vitro diagnosis (e.g. in assays for fatty acids, cholesterol, and the like).